This invention relates to a sheet transporting device for use in an image forming apparatus, such as ink jet printing apparatus for printing images generated by facsimile machine, copying machine, printer, computer output device, and the like.
There have been marketed various types of image forming apparatuses. Recently, ink jet printing apparatuses have been marketed in which ink is emitted from nozzles of an ink emission portion to print an image or letter on a surface of a printing sheet. Further, there has been proposed an ink jet printing apparatus which can print images on both sides of a printing sheet.
For example, Japanese Unexamined Patent Publication No. 5-185661 discloses an ink jet printing apparatus including a first ink emission portion for printing an image on the front side of a printing sheet and a second ink emission portion for printing another image on the back side of the sheet which are arranged along a horizontal plane. In this known apparatus, the printing sheet is transported along the horizontal plane by wide transporting belts having a width greater than the width of printing sheet.
Also, Japanese Unexamined Patent Publication No. 5-330037 discloses an ink jet printing apparatus in which ink emission portions are vertically arranged and opposed to each other with respect to a horizontal plane along which a printing sheet is transported. The opposite ink emission portions print images on both sides of a horizontally transported printing sheet. In this apparatus, too, to support the printing sheet, there is provided wide transporting belts having a width greater than the width of printing sheet prior to and after the printing portion.
In these known apparatuses, the printing sheet is supported by belts entirely extending in a direction of width of the printing sheet to prevent flexure of the printing sheet by the weight. Further, the printing sheet is printed with images on the both sides thereof. Accordingly, when the printing sheet is transferred to such wide transporting belt from the printing portion before the ink completely dries out, printed fresh images are liable to be damaged by the belt, and ink of printed fresh images is also liable to smear the belt.
In view of these problems, in ink jet printing apparatuses operable to print images on both sides of a printing sheet, there has been seriously demanded a way of reliably supporting or holding side ends of a printing sheet to make free a center area of the printing sheet onto which an image is to be printed.